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direct-io.c

/*
 * fs/direct-io.c
 *
 * Copyright (C) 2002, Linus Torvalds.
 *
 * O_DIRECT
 *
 * 04Jul2002      akpm@zip.com.au
 *          Initial version
 * 11Sep2002      janetinc@us.ibm.com
 *          added readv/writev support.
 * 29Oct2002      akpm@zip.com.au
 *          rewrote bio_add_page() support.
 * 30Oct2002      pbadari@us.ibm.com
 *          added support for non-aligned IO.
 * 06Nov2002      pbadari@us.ibm.com
 *          added asynchronous IO support.
 * 21Jul2003      nathans@sgi.com
 *          added IO completion notifier.
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/slab.h>
#include <linux/highmem.h>
#include <linux/pagemap.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/bio.h>
#include <linux/wait.h>
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/buffer_head.h>
#include <linux/rwsem.h>
#include <linux/uio.h>
#include <asm/atomic.h>

/*
 * How many user pages to map in one call to get_user_pages().  This determines
 * the size of a structure on the stack.
 */
#define DIO_PAGES 64

/*
 * This code generally works in units of "dio_blocks".  A dio_block is
 * somewhere between the hard sector size and the filesystem block size.  it
 * is determined on a per-invocation basis.   When talking to the filesystem
 * we need to convert dio_blocks to fs_blocks by scaling the dio_block quantity
 * down by dio->blkfactor.  Similarly, fs-blocksize quantities are converted
 * to bio_block quantities by shifting left by blkfactor.
 *
 * If blkfactor is zero then the user's request was aligned to the filesystem's
 * blocksize.
 *
 * lock_type is DIO_LOCKING for regular files on direct-IO-naive filesystems.
 * This determines whether we need to do the fancy locking which prevents
 * direct-IO from being able to read uninitialised disk blocks.  If its zero
 * (blockdev) this locking is not done, and if it is DIO_OWN_LOCKING i_mutex is
 * not held for the entire direct write (taken briefly, initially, during a
 * direct read though, but its never held for the duration of a direct-IO).
 */

struct dio {
      /* BIO submission state */
      struct bio *bio;        /* bio under assembly */
      struct inode *inode;
      int rw;
      loff_t i_size;                /* i_size when submitted */
      int lock_type;                /* doesn't change */
      unsigned blkbits;       /* doesn't change */
      unsigned blkfactor;           /* When we're using an alignment which
                                 is finer than the filesystem's soft
                                 blocksize, this specifies how much
                                 finer.  blkfactor=2 means 1/4-block
                                 alignment.  Does not change */
      unsigned start_zero_done;     /* flag: sub-blocksize zeroing has
                                 been performed at the start of a
                                 write */
      int pages_in_io;        /* approximate total IO pages */
      size_t      size;             /* total request size (doesn't change)*/
      sector_t block_in_file;       /* Current offset into the underlying
                                 file in dio_block units. */
      unsigned blocks_available;    /* At block_in_file.  changes */
      sector_t final_block_in_request;/* doesn't change */
      unsigned first_block_in_page; /* doesn't change, Used only once */
      int boundary;                 /* prev block is at a boundary */
      int reap_counter;       /* rate limit reaping */
      get_block_t *get_block;       /* block mapping function */
      dio_iodone_t *end_io;         /* IO completion function */
      sector_t final_block_in_bio;  /* current final block in bio + 1 */
      sector_t next_block_for_io;   /* next block to be put under IO,
                                 in dio_blocks units */
      struct buffer_head map_bh;    /* last get_block() result */

      /*
       * Deferred addition of a page to the dio.  These variables are
       * private to dio_send_cur_page(), submit_page_section() and
       * dio_bio_add_page().
       */
      struct page *cur_page;        /* The page */
      unsigned cur_page_offset;     /* Offset into it, in bytes */
      unsigned cur_page_len;        /* Nr of bytes at cur_page_offset */
      sector_t cur_page_block;      /* Where it starts */

      /*
       * Page fetching state. These variables belong to dio_refill_pages().
       */
      int curr_page;                /* changes */
      int total_pages;        /* doesn't change */
      unsigned long curr_user_address;/* changes */

      /*
       * Page queue.  These variables belong to dio_refill_pages() and
       * dio_get_page().
       */
      struct page *pages[DIO_PAGES];      /* page buffer */
      unsigned head;                /* next page to process */
      unsigned tail;                /* last valid page + 1 */
      int page_errors;        /* errno from get_user_pages() */

      /* BIO completion state */
      spinlock_t bio_lock;          /* protects BIO fields below */
      unsigned long refcount;       /* direct_io_worker() and bios */
      struct bio *bio_list;         /* singly linked via bi_private */
      struct task_struct *waiter;   /* waiting task (NULL if none) */

      /* AIO related stuff */
      struct kiocb *iocb;           /* kiocb */
      int is_async;                 /* is IO async ? */
      int io_error;                 /* IO error in completion path */
      ssize_t result;                 /* IO result */
};

/*
 * How many pages are in the queue?
 */
static inline unsigned dio_pages_present(struct dio *dio)
{
      return dio->tail - dio->head;
}

/*
 * Go grab and pin some userspace pages.   Typically we'll get 64 at a time.
 */
static int dio_refill_pages(struct dio *dio)
{
      int ret;
      int nr_pages;

      nr_pages = min(dio->total_pages - dio->curr_page, DIO_PAGES);
      down_read(&current->mm->mmap_sem);
      ret = get_user_pages(
            current,                /* Task for fault acounting */
            current->mm,                  /* whose pages? */
            dio->curr_user_address,       /* Where from? */
            nr_pages,               /* How many pages? */
            dio->rw == READ,        /* Write to memory? */
            0,                      /* force (?) */
            &dio->pages[0],
            NULL);                        /* vmas */
      up_read(&current->mm->mmap_sem);

      if (ret < 0 && dio->blocks_available && (dio->rw & WRITE)) {
            struct page *page = ZERO_PAGE(0);
            /*
             * A memory fault, but the filesystem has some outstanding
             * mapped blocks.  We need to use those blocks up to avoid
             * leaking stale data in the file.
             */
            if (dio->page_errors == 0)
                  dio->page_errors = ret;
            page_cache_get(page);
            dio->pages[0] = page;
            dio->head = 0;
            dio->tail = 1;
            ret = 0;
            goto out;
      }

      if (ret >= 0) {
            dio->curr_user_address += ret * PAGE_SIZE;
            dio->curr_page += ret;
            dio->head = 0;
            dio->tail = ret;
            ret = 0;
      }
out:
      return ret; 
}

/*
 * Get another userspace page.  Returns an ERR_PTR on error.  Pages are
 * buffered inside the dio so that we can call get_user_pages() against a
 * decent number of pages, less frequently.  To provide nicer use of the
 * L1 cache.
 */
static struct page *dio_get_page(struct dio *dio)
{
      if (dio_pages_present(dio) == 0) {
            int ret;

            ret = dio_refill_pages(dio);
            if (ret)
                  return ERR_PTR(ret);
            BUG_ON(dio_pages_present(dio) == 0);
      }
      return dio->pages[dio->head++];
}

/**
 * dio_complete() - called when all DIO BIO I/O has been completed
 * @offset: the byte offset in the file of the completed operation
 *
 * This releases locks as dictated by the locking type, lets interested parties
 * know that a DIO operation has completed, and calculates the resulting return
 * code for the operation.
 *
 * It lets the filesystem know if it registered an interest earlier via
 * get_block.  Pass the private field of the map buffer_head so that
 * filesystems can use it to hold additional state between get_block calls and
 * dio_complete.
 */
static int dio_complete(struct dio *dio, loff_t offset, int ret)
{
      ssize_t transferred = 0;

      /*
       * AIO submission can race with bio completion to get here while
       * expecting to have the last io completed by bio completion.
       * In that case -EIOCBQUEUED is in fact not an error we want
       * to preserve through this call.
       */
      if (ret == -EIOCBQUEUED)
            ret = 0;

      if (dio->result) {
            transferred = dio->result;

            /* Check for short read case */
            if ((dio->rw == READ) && ((offset + transferred) > dio->i_size))
                  transferred = dio->i_size - offset;
      }

      if (dio->end_io && dio->result)
            dio->end_io(dio->iocb, offset, transferred,
                      dio->map_bh.b_private);
      if (dio->lock_type == DIO_LOCKING)
            /* lockdep: non-owner release */
            up_read_non_owner(&dio->inode->i_alloc_sem);

      if (ret == 0)
            ret = dio->page_errors;
      if (ret == 0)
            ret = dio->io_error;
      if (ret == 0)
            ret = transferred;

      return ret;
}

static int dio_bio_complete(struct dio *dio, struct bio *bio);
/*
 * Asynchronous IO callback. 
 */
static void dio_bio_end_aio(struct bio *bio, int error)
{
      struct dio *dio = bio->bi_private;
      unsigned long remaining;
      unsigned long flags;

      /* cleanup the bio */
      dio_bio_complete(dio, bio);

      spin_lock_irqsave(&dio->bio_lock, flags);
      remaining = --dio->refcount;
      if (remaining == 1 && dio->waiter)
            wake_up_process(dio->waiter);
      spin_unlock_irqrestore(&dio->bio_lock, flags);

      if (remaining == 0) {
            int ret = dio_complete(dio, dio->iocb->ki_pos, 0);
            aio_complete(dio->iocb, ret, 0);
            kfree(dio);
      }
}

/*
 * The BIO completion handler simply queues the BIO up for the process-context
 * handler.
 *
 * During I/O bi_private points at the dio.  After I/O, bi_private is used to
 * implement a singly-linked list of completed BIOs, at dio->bio_list.
 */
static void dio_bio_end_io(struct bio *bio, int error)
{
      struct dio *dio = bio->bi_private;
      unsigned long flags;

      spin_lock_irqsave(&dio->bio_lock, flags);
      bio->bi_private = dio->bio_list;
      dio->bio_list = bio;
      if (--dio->refcount == 1 && dio->waiter)
            wake_up_process(dio->waiter);
      spin_unlock_irqrestore(&dio->bio_lock, flags);
}

static int
dio_bio_alloc(struct dio *dio, struct block_device *bdev,
            sector_t first_sector, int nr_vecs)
{
      struct bio *bio;

      bio = bio_alloc(GFP_KERNEL, nr_vecs);
      if (bio == NULL)
            return -ENOMEM;

      bio->bi_bdev = bdev;
      bio->bi_sector = first_sector;
      if (dio->is_async)
            bio->bi_end_io = dio_bio_end_aio;
      else
            bio->bi_end_io = dio_bio_end_io;

      dio->bio = bio;
      return 0;
}

/*
 * In the AIO read case we speculatively dirty the pages before starting IO.
 * During IO completion, any of these pages which happen to have been written
 * back will be redirtied by bio_check_pages_dirty().
 *
 * bios hold a dio reference between submit_bio and ->end_io.
 */
static void dio_bio_submit(struct dio *dio)
{
      struct bio *bio = dio->bio;
      unsigned long flags;

      bio->bi_private = dio;

      spin_lock_irqsave(&dio->bio_lock, flags);
      dio->refcount++;
      spin_unlock_irqrestore(&dio->bio_lock, flags);

      if (dio->is_async && dio->rw == READ)
            bio_set_pages_dirty(bio);

      submit_bio(dio->rw, bio);

      dio->bio = NULL;
      dio->boundary = 0;
}

/*
 * Release any resources in case of a failure
 */
static void dio_cleanup(struct dio *dio)
{
      while (dio_pages_present(dio))
            page_cache_release(dio_get_page(dio));
}

/*
 * Wait for the next BIO to complete.  Remove it and return it.  NULL is
 * returned once all BIOs have been completed.  This must only be called once
 * all bios have been issued so that dio->refcount can only decrease.  This
 * requires that that the caller hold a reference on the dio.
 */
static struct bio *dio_await_one(struct dio *dio)
{
      unsigned long flags;
      struct bio *bio = NULL;

      spin_lock_irqsave(&dio->bio_lock, flags);

      /*
       * Wait as long as the list is empty and there are bios in flight.  bio
       * completion drops the count, maybe adds to the list, and wakes while
       * holding the bio_lock so we don't need set_current_state()'s barrier
       * and can call it after testing our condition.
       */
      while (dio->refcount > 1 && dio->bio_list == NULL) {
            __set_current_state(TASK_UNINTERRUPTIBLE);
            dio->waiter = current;
            spin_unlock_irqrestore(&dio->bio_lock, flags);
            io_schedule();
            /* wake up sets us TASK_RUNNING */
            spin_lock_irqsave(&dio->bio_lock, flags);
            dio->waiter = NULL;
      }
      if (dio->bio_list) {
            bio = dio->bio_list;
            dio->bio_list = bio->bi_private;
      }
      spin_unlock_irqrestore(&dio->bio_lock, flags);
      return bio;
}

/*
 * Process one completed BIO.  No locks are held.
 */
static int dio_bio_complete(struct dio *dio, struct bio *bio)
{
      const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
      struct bio_vec *bvec = bio->bi_io_vec;
      int page_no;

      if (!uptodate)
            dio->io_error = -EIO;

      if (dio->is_async && dio->rw == READ) {
            bio_check_pages_dirty(bio);   /* transfers ownership */
      } else {
            for (page_no = 0; page_no < bio->bi_vcnt; page_no++) {
                  struct page *page = bvec[page_no].bv_page;

                  if (dio->rw == READ && !PageCompound(page))
                        set_page_dirty_lock(page);
                  page_cache_release(page);
            }
            bio_put(bio);
      }
      return uptodate ? 0 : -EIO;
}

/*
 * Wait on and process all in-flight BIOs.  This must only be called once
 * all bios have been issued so that the refcount can only decrease.
 * This just waits for all bios to make it through dio_bio_complete.  IO
 * errors are propagated through dio->io_error and should be propagated via
 * dio_complete().
 */
static void dio_await_completion(struct dio *dio)
{
      struct bio *bio;
      do {
            bio = dio_await_one(dio);
            if (bio)
                  dio_bio_complete(dio, bio);
      } while (bio);
}

/*
 * A really large O_DIRECT read or write can generate a lot of BIOs.  So
 * to keep the memory consumption sane we periodically reap any completed BIOs
 * during the BIO generation phase.
 *
 * This also helps to limit the peak amount of pinned userspace memory.
 */
static int dio_bio_reap(struct dio *dio)
{
      int ret = 0;

      if (dio->reap_counter++ >= 64) {
            while (dio->bio_list) {
                  unsigned long flags;
                  struct bio *bio;
                  int ret2;

                  spin_lock_irqsave(&dio->bio_lock, flags);
                  bio = dio->bio_list;
                  dio->bio_list = bio->bi_private;
                  spin_unlock_irqrestore(&dio->bio_lock, flags);
                  ret2 = dio_bio_complete(dio, bio);
                  if (ret == 0)
                        ret = ret2;
            }
            dio->reap_counter = 0;
      }
      return ret;
}

/*
 * Call into the fs to map some more disk blocks.  We record the current number
 * of available blocks at dio->blocks_available.  These are in units of the
 * fs blocksize, (1 << inode->i_blkbits).
 *
 * The fs is allowed to map lots of blocks at once.  If it wants to do that,
 * it uses the passed inode-relative block number as the file offset, as usual.
 *
 * get_block() is passed the number of i_blkbits-sized blocks which direct_io
 * has remaining to do.  The fs should not map more than this number of blocks.
 *
 * If the fs has mapped a lot of blocks, it should populate bh->b_size to
 * indicate how much contiguous disk space has been made available at
 * bh->b_blocknr.
 *
 * If *any* of the mapped blocks are new, then the fs must set buffer_new().
 * This isn't very efficient...
 *
 * In the case of filesystem holes: the fs may return an arbitrarily-large
 * hole by returning an appropriate value in b_size and by clearing
 * buffer_mapped().  However the direct-io code will only process holes one
 * block at a time - it will repeatedly call get_block() as it walks the hole.
 */
static int get_more_blocks(struct dio *dio)
{
      int ret;
      struct buffer_head *map_bh = &dio->map_bh;
      sector_t fs_startblk;   /* Into file, in filesystem-sized blocks */
      unsigned long fs_count; /* Number of filesystem-sized blocks */
      unsigned long dio_count;/* Number of dio_block-sized blocks */
      unsigned long blkmask;
      int create;

      /*
       * If there was a memory error and we've overwritten all the
       * mapped blocks then we can now return that memory error
       */
      ret = dio->page_errors;
      if (ret == 0) {
            BUG_ON(dio->block_in_file >= dio->final_block_in_request);
            fs_startblk = dio->block_in_file >> dio->blkfactor;
            dio_count = dio->final_block_in_request - dio->block_in_file;
            fs_count = dio_count >> dio->blkfactor;
            blkmask = (1 << dio->blkfactor) - 1;
            if (dio_count & blkmask)      
                  fs_count++;

            map_bh->b_state = 0;
            map_bh->b_size = fs_count << dio->inode->i_blkbits;

            create = dio->rw & WRITE;
            if (dio->lock_type == DIO_LOCKING) {
                  if (dio->block_in_file < (i_size_read(dio->inode) >>
                                          dio->blkbits))
                        create = 0;
            } else if (dio->lock_type == DIO_NO_LOCKING) {
                  create = 0;
            }

            /*
             * For writes inside i_size we forbid block creations: only
             * overwrites are permitted.  We fall back to buffered writes
             * at a higher level for inside-i_size block-instantiating
             * writes.
             */
            ret = (*dio->get_block)(dio->inode, fs_startblk,
                                    map_bh, create);
      }
      return ret;
}

/*
 * There is no bio.  Make one now.
 */
static int dio_new_bio(struct dio *dio, sector_t start_sector)
{
      sector_t sector;
      int ret, nr_pages;

      ret = dio_bio_reap(dio);
      if (ret)
            goto out;
      sector = start_sector << (dio->blkbits - 9);
      nr_pages = min(dio->pages_in_io, bio_get_nr_vecs(dio->map_bh.b_bdev));
      BUG_ON(nr_pages <= 0);
      ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
      dio->boundary = 0;
out:
      return ret;
}

/*
 * Attempt to put the current chunk of 'cur_page' into the current BIO.  If
 * that was successful then update final_block_in_bio and take a ref against
 * the just-added page.
 *
 * Return zero on success.  Non-zero means the caller needs to start a new BIO.
 */
static int dio_bio_add_page(struct dio *dio)
{
      int ret;

      ret = bio_add_page(dio->bio, dio->cur_page,
                  dio->cur_page_len, dio->cur_page_offset);
      if (ret == dio->cur_page_len) {
            /*
             * Decrement count only, if we are done with this page
             */
            if ((dio->cur_page_len + dio->cur_page_offset) == PAGE_SIZE)
                  dio->pages_in_io--;
            page_cache_get(dio->cur_page);
            dio->final_block_in_bio = dio->cur_page_block +
                  (dio->cur_page_len >> dio->blkbits);
            ret = 0;
      } else {
            ret = 1;
      }
      return ret;
}
            
/*
 * Put cur_page under IO.  The section of cur_page which is described by
 * cur_page_offset,cur_page_len is put into a BIO.  The section of cur_page
 * starts on-disk at cur_page_block.
 *
 * We take a ref against the page here (on behalf of its presence in the bio).
 *
 * The caller of this function is responsible for removing cur_page from the
 * dio, and for dropping the refcount which came from that presence.
 */
static int dio_send_cur_page(struct dio *dio)
{
      int ret = 0;

      if (dio->bio) {
            /*
             * See whether this new request is contiguous with the old
             */
            if (dio->final_block_in_bio != dio->cur_page_block)
                  dio_bio_submit(dio);
            /*
             * Submit now if the underlying fs is about to perform a
             * metadata read
             */
            if (dio->boundary)
                  dio_bio_submit(dio);
      }

      if (dio->bio == NULL) {
            ret = dio_new_bio(dio, dio->cur_page_block);
            if (ret)
                  goto out;
      }

      if (dio_bio_add_page(dio) != 0) {
            dio_bio_submit(dio);
            ret = dio_new_bio(dio, dio->cur_page_block);
            if (ret == 0) {
                  ret = dio_bio_add_page(dio);
                  BUG_ON(ret != 0);
            }
      }
out:
      return ret;
}

/*
 * An autonomous function to put a chunk of a page under deferred IO.
 *
 * The caller doesn't actually know (or care) whether this piece of page is in
 * a BIO, or is under IO or whatever.  We just take care of all possible 
 * situations here.  The separation between the logic of do_direct_IO() and
 * that of submit_page_section() is important for clarity.  Please don't break.
 *
 * The chunk of page starts on-disk at blocknr.
 *
 * We perform deferred IO, by recording the last-submitted page inside our
 * private part of the dio structure.  If possible, we just expand the IO
 * across that page here.
 *
 * If that doesn't work out then we put the old page into the bio and add this
 * page to the dio instead.
 */
static int
submit_page_section(struct dio *dio, struct page *page,
            unsigned offset, unsigned len, sector_t blocknr)
{
      int ret = 0;

      if (dio->rw & WRITE) {
            /*
             * Read accounting is performed in submit_bio()
             */
            task_io_account_write(len);
      }

      /*
       * Can we just grow the current page's presence in the dio?
       */
      if (  (dio->cur_page == page) &&
            (dio->cur_page_offset + dio->cur_page_len == offset) &&
            (dio->cur_page_block +
                  (dio->cur_page_len >> dio->blkbits) == blocknr)) {
            dio->cur_page_len += len;

            /*
             * If dio->boundary then we want to schedule the IO now to
             * avoid metadata seeks.
             */
            if (dio->boundary) {
                  ret = dio_send_cur_page(dio);
                  page_cache_release(dio->cur_page);
                  dio->cur_page = NULL;
            }
            goto out;
      }

      /*
       * If there's a deferred page already there then send it.
       */
      if (dio->cur_page) {
            ret = dio_send_cur_page(dio);
            page_cache_release(dio->cur_page);
            dio->cur_page = NULL;
            if (ret)
                  goto out;
      }

      page_cache_get(page);         /* It is in dio */
      dio->cur_page = page;
      dio->cur_page_offset = offset;
      dio->cur_page_len = len;
      dio->cur_page_block = blocknr;
out:
      return ret;
}

/*
 * Clean any dirty buffers in the blockdev mapping which alias newly-created
 * file blocks.  Only called for S_ISREG files - blockdevs do not set
 * buffer_new
 */
static void clean_blockdev_aliases(struct dio *dio)
{
      unsigned i;
      unsigned nblocks;

      nblocks = dio->map_bh.b_size >> dio->inode->i_blkbits;

      for (i = 0; i < nblocks; i++) {
            unmap_underlying_metadata(dio->map_bh.b_bdev,
                              dio->map_bh.b_blocknr + i);
      }
}

/*
 * If we are not writing the entire block and get_block() allocated
 * the block for us, we need to fill-in the unused portion of the
 * block with zeros. This happens only if user-buffer, fileoffset or
 * io length is not filesystem block-size multiple.
 *
 * `end' is zero if we're doing the start of the IO, 1 at the end of the
 * IO.
 */
static void dio_zero_block(struct dio *dio, int end)
{
      unsigned dio_blocks_per_fs_block;
      unsigned this_chunk_blocks;   /* In dio_blocks */
      unsigned this_chunk_bytes;
      struct page *page;

      dio->start_zero_done = 1;
      if (!dio->blkfactor || !buffer_new(&dio->map_bh))
            return;

      dio_blocks_per_fs_block = 1 << dio->blkfactor;
      this_chunk_blocks = dio->block_in_file & (dio_blocks_per_fs_block - 1);

      if (!this_chunk_blocks)
            return;

      /*
       * We need to zero out part of an fs block.  It is either at the
       * beginning or the end of the fs block.
       */
      if (end) 
            this_chunk_blocks = dio_blocks_per_fs_block - this_chunk_blocks;

      this_chunk_bytes = this_chunk_blocks << dio->blkbits;

      page = ZERO_PAGE(0);
      if (submit_page_section(dio, page, 0, this_chunk_bytes, 
                        dio->next_block_for_io))
            return;

      dio->next_block_for_io += this_chunk_blocks;
}

/*
 * Walk the user pages, and the file, mapping blocks to disk and generating
 * a sequence of (page,offset,len,block) mappings.  These mappings are injected
 * into submit_page_section(), which takes care of the next stage of submission
 *
 * Direct IO against a blockdev is different from a file.  Because we can
 * happily perform page-sized but 512-byte aligned IOs.  It is important that
 * blockdev IO be able to have fine alignment and large sizes.
 *
 * So what we do is to permit the ->get_block function to populate bh.b_size
 * with the size of IO which is permitted at this offset and this i_blkbits.
 *
 * For best results, the blockdev should be set up with 512-byte i_blkbits and
 * it should set b_size to PAGE_SIZE or more inside get_block().  This gives
 * fine alignment but still allows this function to work in PAGE_SIZE units.
 */
static int do_direct_IO(struct dio *dio)
{
      const unsigned blkbits = dio->blkbits;
      const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
      struct page *page;
      unsigned block_in_page;
      struct buffer_head *map_bh = &dio->map_bh;
      int ret = 0;

      /* The I/O can start at any block offset within the first page */
      block_in_page = dio->first_block_in_page;

      while (dio->block_in_file < dio->final_block_in_request) {
            page = dio_get_page(dio);
            if (IS_ERR(page)) {
                  ret = PTR_ERR(page);
                  goto out;
            }

            while (block_in_page < blocks_per_page) {
                  unsigned offset_in_page = block_in_page << blkbits;
                  unsigned this_chunk_bytes;    /* # of bytes mapped */
                  unsigned this_chunk_blocks;   /* # of blocks */
                  unsigned u;

                  if (dio->blocks_available == 0) {
                        /*
                         * Need to go and map some more disk
                         */
                        unsigned long blkmask;
                        unsigned long dio_remainder;

                        ret = get_more_blocks(dio);
                        if (ret) {
                              page_cache_release(page);
                              goto out;
                        }
                        if (!buffer_mapped(map_bh))
                              goto do_holes;

                        dio->blocks_available =
                                    map_bh->b_size >> dio->blkbits;
                        dio->next_block_for_io =
                              map_bh->b_blocknr << dio->blkfactor;
                        if (buffer_new(map_bh))
                              clean_blockdev_aliases(dio);

                        if (!dio->blkfactor)
                              goto do_holes;

                        blkmask = (1 << dio->blkfactor) - 1;
                        dio_remainder = (dio->block_in_file & blkmask);

                        /*
                         * If we are at the start of IO and that IO
                         * starts partway into a fs-block,
                         * dio_remainder will be non-zero.  If the IO
                         * is a read then we can simply advance the IO
                         * cursor to the first block which is to be
                         * read.  But if the IO is a write and the
                         * block was newly allocated we cannot do that;
                         * the start of the fs block must be zeroed out
                         * on-disk
                         */
                        if (!buffer_new(map_bh))
                              dio->next_block_for_io += dio_remainder;
                        dio->blocks_available -= dio_remainder;
                  }
do_holes:
                  /* Handle holes */
                  if (!buffer_mapped(map_bh)) {
                        loff_t i_size_aligned;

                        /* AKPM: eargh, -ENOTBLK is a hack */
                        if (dio->rw & WRITE) {
                              page_cache_release(page);
                              return -ENOTBLK;
                        }

                        /*
                         * Be sure to account for a partial block as the
                         * last block in the file
                         */
                        i_size_aligned = ALIGN(i_size_read(dio->inode),
                                          1 << blkbits);
                        if (dio->block_in_file >=
                                    i_size_aligned >> blkbits) {
                              /* We hit eof */
                              page_cache_release(page);
                              goto out;
                        }
                        zero_user_page(page, block_in_page << blkbits,
                                    1 << blkbits, KM_USER0);
                        dio->block_in_file++;
                        block_in_page++;
                        goto next_block;
                  }

                  /*
                   * If we're performing IO which has an alignment which
                   * is finer than the underlying fs, go check to see if
                   * we must zero out the start of this block.
                   */
                  if (unlikely(dio->blkfactor && !dio->start_zero_done))
                        dio_zero_block(dio, 0);

                  /*
                   * Work out, in this_chunk_blocks, how much disk we
                   * can add to this page
                   */
                  this_chunk_blocks = dio->blocks_available;
                  u = (PAGE_SIZE - offset_in_page) >> blkbits;
                  if (this_chunk_blocks > u)
                        this_chunk_blocks = u;
                  u = dio->final_block_in_request - dio->block_in_file;
                  if (this_chunk_blocks > u)
                        this_chunk_blocks = u;
                  this_chunk_bytes = this_chunk_blocks << blkbits;
                  BUG_ON(this_chunk_bytes == 0);

                  dio->boundary = buffer_boundary(map_bh);
                  ret = submit_page_section(dio, page, offset_in_page,
                        this_chunk_bytes, dio->next_block_for_io);
                  if (ret) {
                        page_cache_release(page);
                        goto out;
                  }
                  dio->next_block_for_io += this_chunk_blocks;

                  dio->block_in_file += this_chunk_blocks;
                  block_in_page += this_chunk_blocks;
                  dio->blocks_available -= this_chunk_blocks;
next_block:
                  BUG_ON(dio->block_in_file > dio->final_block_in_request);
                  if (dio->block_in_file == dio->final_block_in_request)
                        break;
            }

            /* Drop the ref which was taken in get_user_pages() */
            page_cache_release(page);
            block_in_page = 0;
      }
out:
      return ret;
}

/*
 * Releases both i_mutex and i_alloc_sem
 */
static ssize_t
direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode, 
      const struct iovec *iov, loff_t offset, unsigned long nr_segs, 
      unsigned blkbits, get_block_t get_block, dio_iodone_t end_io,
      struct dio *dio)
{
      unsigned long user_addr; 
      unsigned long flags;
      int seg;
      ssize_t ret = 0;
      ssize_t ret2;
      size_t bytes;

      dio->inode = inode;
      dio->rw = rw;
      dio->blkbits = blkbits;
      dio->blkfactor = inode->i_blkbits - blkbits;
      dio->block_in_file = offset >> blkbits;

      dio->get_block = get_block;
      dio->end_io = end_io;
      dio->final_block_in_bio = -1;
      dio->next_block_for_io = -1;

      dio->iocb = iocb;
      dio->i_size = i_size_read(inode);

      spin_lock_init(&dio->bio_lock);
      dio->refcount = 1;

      /*
       * In case of non-aligned buffers, we may need 2 more
       * pages since we need to zero out first and last block.
       */
      if (unlikely(dio->blkfactor))
            dio->pages_in_io = 2;

      for (seg = 0; seg < nr_segs; seg++) {
            user_addr = (unsigned long)iov[seg].iov_base;
            dio->pages_in_io +=
                  ((user_addr+iov[seg].iov_len +PAGE_SIZE-1)/PAGE_SIZE
                        - user_addr/PAGE_SIZE);
      }

      for (seg = 0; seg < nr_segs; seg++) {
            user_addr = (unsigned long)iov[seg].iov_base;
            dio->size += bytes = iov[seg].iov_len;

            /* Index into the first page of the first block */
            dio->first_block_in_page = (user_addr & ~PAGE_MASK) >> blkbits;
            dio->final_block_in_request = dio->block_in_file +
                                    (bytes >> blkbits);
            /* Page fetching state */
            dio->head = 0;
            dio->tail = 0;
            dio->curr_page = 0;

            dio->total_pages = 0;
            if (user_addr & (PAGE_SIZE-1)) {
                  dio->total_pages++;
                  bytes -= PAGE_SIZE - (user_addr & (PAGE_SIZE - 1));
            }
            dio->total_pages += (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
            dio->curr_user_address = user_addr;
      
            ret = do_direct_IO(dio);

            dio->result += iov[seg].iov_len -
                  ((dio->final_block_in_request - dio->block_in_file) <<
                              blkbits);

            if (ret) {
                  dio_cleanup(dio);
                  break;
            }
      } /* end iovec loop */

      if (ret == -ENOTBLK && (rw & WRITE)) {
            /*
             * The remaining part of the request will be
             * be handled by buffered I/O when we return
             */
            ret = 0;
      }
      /*
       * There may be some unwritten disk at the end of a part-written
       * fs-block-sized block.  Go zero that now.
       */
      dio_zero_block(dio, 1);

      if (dio->cur_page) {
            ret2 = dio_send_cur_page(dio);
            if (ret == 0)
                  ret = ret2;
            page_cache_release(dio->cur_page);
            dio->cur_page = NULL;
      }
      if (dio->bio)
            dio_bio_submit(dio);

      /* All IO is now issued, send it on its way */
      blk_run_address_space(inode->i_mapping);

      /*
       * It is possible that, we return short IO due to end of file.
       * In that case, we need to release all the pages we got hold on.
       */
      dio_cleanup(dio);

      /*
       * All block lookups have been performed. For READ requests
       * we can let i_mutex go now that its achieved its purpose
       * of protecting us from looking up uninitialized blocks.
       */
      if ((rw == READ) && (dio->lock_type == DIO_LOCKING))
            mutex_unlock(&dio->inode->i_mutex);

      /*
       * The only time we want to leave bios in flight is when a successful
       * partial aio read or full aio write have been setup.  In that case
       * bio completion will call aio_complete.  The only time it's safe to
       * call aio_complete is when we return -EIOCBQUEUED, so we key on that.
       * This had *better* be the only place that raises -EIOCBQUEUED.
       */
      BUG_ON(ret == -EIOCBQUEUED);
      if (dio->is_async && ret == 0 && dio->result &&
          ((rw & READ) || (dio->result == dio->size)))
            ret = -EIOCBQUEUED;

      if (ret != -EIOCBQUEUED)
            dio_await_completion(dio);

      /*
       * Sync will always be dropping the final ref and completing the
       * operation.  AIO can if it was a broken operation described above or
       * in fact if all the bios race to complete before we get here.  In
       * that case dio_complete() translates the EIOCBQUEUED into the proper
       * return code that the caller will hand to aio_complete().
       *
       * This is managed by the bio_lock instead of being an atomic_t so that
       * completion paths can drop their ref and use the remaining count to
       * decide to wake the submission path atomically.
       */
      spin_lock_irqsave(&dio->bio_lock, flags);
      ret2 = --dio->refcount;
      spin_unlock_irqrestore(&dio->bio_lock, flags);

      if (ret2 == 0) {
            ret = dio_complete(dio, offset, ret);
            kfree(dio);
      } else
            BUG_ON(ret != -EIOCBQUEUED);

      return ret;
}

/*
 * This is a library function for use by filesystem drivers.
 * The locking rules are governed by the dio_lock_type parameter.
 *
 * DIO_NO_LOCKING (no locking, for raw block device access)
 * For writes, i_mutex is not held on entry; it is never taken.
 *
 * DIO_LOCKING (simple locking for regular files)
 * For writes we are called under i_mutex and return with i_mutex held, even
 * though it is internally dropped.
 * For reads, i_mutex is not held on entry, but it is taken and dropped before
 * returning.
 *
 * DIO_OWN_LOCKING (filesystem provides synchronisation and handling of
 *    uninitialised data, allowing parallel direct readers and writers)
 * For writes we are called without i_mutex, return without it, never touch it.
 * For reads we are called under i_mutex and return with i_mutex held, even
 * though it may be internally dropped.
 *
 * Additional i_alloc_sem locking requirements described inline below.
 */
ssize_t
__blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
      struct block_device *bdev, const struct iovec *iov, loff_t offset, 
      unsigned long nr_segs, get_block_t get_block, dio_iodone_t end_io,
      int dio_lock_type)
{
      int seg;
      size_t size;
      unsigned long addr;
      unsigned blkbits = inode->i_blkbits;
      unsigned bdev_blkbits = 0;
      unsigned blocksize_mask = (1 << blkbits) - 1;
      ssize_t retval = -EINVAL;
      loff_t end = offset;
      struct dio *dio;
      int release_i_mutex = 0;
      int acquire_i_mutex = 0;

      if (rw & WRITE)
            rw = WRITE_SYNC;

      if (bdev)
            bdev_blkbits = blksize_bits(bdev_hardsect_size(bdev));

      if (offset & blocksize_mask) {
            if (bdev)
                   blkbits = bdev_blkbits;
            blocksize_mask = (1 << blkbits) - 1;
            if (offset & blocksize_mask)
                  goto out;
      }

      /* Check the memory alignment.  Blocks cannot straddle pages */
      for (seg = 0; seg < nr_segs; seg++) {
            addr = (unsigned long)iov[seg].iov_base;
            size = iov[seg].iov_len;
            end += size;
            if ((addr & blocksize_mask) || (size & blocksize_mask))  {
                  if (bdev)
                         blkbits = bdev_blkbits;
                  blocksize_mask = (1 << blkbits) - 1;
                  if ((addr & blocksize_mask) || (size & blocksize_mask))  
                        goto out;
            }
      }

      dio = kzalloc(sizeof(*dio), GFP_KERNEL);
      retval = -ENOMEM;
      if (!dio)
            goto out;

      /*
       * For block device access DIO_NO_LOCKING is used,
       *    neither readers nor writers do any locking at all
       * For regular files using DIO_LOCKING,
       *    readers need to grab i_mutex and i_alloc_sem
       *    writers need to grab i_alloc_sem only (i_mutex is already held)
       * For regular files using DIO_OWN_LOCKING,
       *    neither readers nor writers take any locks here
       */
      dio->lock_type = dio_lock_type;
      if (dio_lock_type != DIO_NO_LOCKING) {
            /* watch out for a 0 len io from a tricksy fs */
            if (rw == READ && end > offset) {
                  struct address_space *mapping;

                  mapping = iocb->ki_filp->f_mapping;
                  if (dio_lock_type != DIO_OWN_LOCKING) {
                        mutex_lock(&inode->i_mutex);
                        release_i_mutex = 1;
                  }

                  retval = filemap_write_and_wait_range(mapping, offset,
                                                end - 1);
                  if (retval) {
                        kfree(dio);
                        goto out;
                  }

                  if (dio_lock_type == DIO_OWN_LOCKING) {
                        mutex_unlock(&inode->i_mutex);
                        acquire_i_mutex = 1;
                  }
            }

            if (dio_lock_type == DIO_LOCKING)
                  /* lockdep: not the owner will release it */
                  down_read_non_owner(&inode->i_alloc_sem);
      }

      /*
       * For file extending writes updating i_size before data
       * writeouts complete can expose uninitialized blocks. So
       * even for AIO, we need to wait for i/o to complete before
       * returning in this case.
       */
      dio->is_async = !is_sync_kiocb(iocb) && !((rw & WRITE) &&
            (end > i_size_read(inode)));

      retval = direct_io_worker(rw, iocb, inode, iov, offset,
                        nr_segs, blkbits, get_block, end_io, dio);

      if (rw == READ && dio_lock_type == DIO_LOCKING)
            release_i_mutex = 0;

out:
      if (release_i_mutex)
            mutex_unlock(&inode->i_mutex);
      else if (acquire_i_mutex)
            mutex_lock(&inode->i_mutex);
      return retval;
}
EXPORT_SYMBOL(__blockdev_direct_IO);

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